It is well known that rubber-reinforced polymers of monovinylidene aromatic compounds, such as styrene, alpha-methylstyrene and ring-substituted styrene, are useful for a variety of purposes. For example, rubber-reinforced styrene polymers having discrete particles of a cross-linked rubber, e.g., polybutadiene, dispersed throughout the styrene polymer matrix can be used in a variety of diverse applications including refrigerator linings, packaging applications, furniture, household appliances and toys. Such rubber-reinforced polymers are commonly referred to as "high impact polystyrene" or "HIPS".
One known process for producing HIPS polymers is the batch or suspension process wherein polymerization occurs within a single reaction vessel. An advantage of the batch process is the ease with which HIPS production can be controlled and monitored due to the use of a single reaction vessel throughout the entire polymerization process. However, the batch process suffers from a number of inherent disadvantages, such as low yields and extended down-time periods, which render it unsuitable for commercial purposes.
In order to overcome the commercial difficulties of the batch process, several continuous flow processes for the production of HIPS have been proposed. Such known processes employ a plurality of serially arranged reaction vessels wherein the degree of polymerization from one vessel to the next. See, for example, U.S. Pat. No. 3,658,946 to Bronstert et al., U.S. Pat. No. 3,243,481 to Ruffing et al., U. S. Pat. No. 4,451,612 to Wang et al. and U.S. Pat. No. 4,567,232 to Echte et al.
Several of these continuous process patents generally suggest that some type of devolatilization zone and recycle pathway be used for returning solvents and unreacted monomer to one of the polymerization reaction zones. However, the recycle stream leaving the devolatilization zone contains a substantial amount of impurities, which can adversely affect the continuous HIPS production process when the impurities are introduced into a polymerization zone. In fact, these impurities can destroy the effectiveness of a free radical initiator and inhibit polymerization for a significant period of time, even indefinitely, when the recycle stream is fed into the initial polymerization reacton zone. Further, such impurities can impart undesirable physical characteristics, such as discoloration, to the HIPS polymers.
None of the above patents recognizes either the cause or the effects of recycle stream impurities. Similarly, none of those patents provides any means for overcoming the adverse effects of these impurities upon a continuous flow process for producing HIPS polymers.